These applications are particularly applicable to scratch cards for phone-cards and lottery tickets, vouchers and other items of value where a PIN number is protected by a scratch-off foil and revealed by scratching off the foil.
Devices based on the principle of optical diffraction are often used for anti-counterfeit and security purposes because they can produce, by the process of optical diffraction, an optically variable image with characteristic features such as depth and parallax (holograms) and movement features and image switches (purely diffraction grating devices and some holographic devices). Such diffractive, optically variable image forming devices are used as anti-counterfeit devices both because their effects are highly recognisable and cannot be duplicated by print technologies, and because specific and difficult to replicate optical and engineering techniques are required for their production.
These diffractive optically variable image forming devices form their effects base on holographic or pure diffraction grating techniques and are often manufactured as embossed surface relief structures as known in the art (for example, Graham Saxby, Practical Holography Prentice Hall 1988). They are typically applied to documents of value, plastic cards and articles of value to be protected in the form of holographic or diffractive hot stamping foil applied to a numbered paper or plastic card base using the known method from the printing industry of the technique of hot stamping of decorative foils.
There are various forms of pure diffraction grating devices in use as such security devices, examples include U.S. Pat. No. 4,568,141, which reveals a diffraction optical authenticating element and U.S. Pat. No. 5,034,003 which reveals another form of optical security device using diffraction gratings. Another form of pure diffraction grating security device can be produced by direct writing by an electron beam and examples are WO 9318419, WO 9504948 and WO 9502200. Teachings on methods of origination useful for dot screen hidden image for example using electron beam lithography can be found in PCT/GB/2002/003257, useful teachings on methods of recording and replaying covert laser readable features using both laser interference and direct write origination can be found in WO 02/03323 A1, WO 02/02351 A1 and WO 02/03109 A1 and useful methods for originating achromatic structures can be found in WO 02/06858 A2—the teachings of all which are incorporated by reference. Diffractive optical variable image forming devices can also be produced by holographic methods and are known by their use in security applications for example on credit cards, secure documents and product authentication—examples of such teachings are U.S. Pat. No. 5,694,229, U.S. Pat. No. 5,483,363, WO 995903.
Currently various approaches have been used for securing PIN data on product and documents. These include tamper evident security scratch-off foils and scratch-off security labelling using anti-counterfeit features such as holograms, diffractive devices, or various forms of security print.
It is often advantageous and known in the art to protect hidden numbers on pre-pay cards or lottery tickets by scratch-off ink systems, where the ink covering the ticket or validation number can be scratched-off using for example a coin but which keeps the number or information completely hidden until usage. Scratch-off diffractive and holographic hot stamping foil is used in this type of application to protect information of value from prior disclosure or alteration—with the advantage that the holographic foil is hard to replicate or obtain and therefore protects the number from alteration or prior use and re-sale. In these applications the hot stamping device would be opaque to prevent viewing of underlying data—usually metallised with aluminium or a similar reflective and opaque metal which can be scratched off to reveal the data. Scratch off ink systems can be combined with an overlaid metallic or holographic metallic layer and some types are described for example in WO 2006/005156 A1 (Taylor et al) which describes a scratch card system consisting of a secure number overlaid by a printed varnish, a soft adhesive and with a metallic or holographic foil applied on top, one method of application being foil stamping or cold foil transfer. Other such systems are described in CA 2 471 024 A1 (Hamilton et al).
Another class of scratch-off protection is provided by scratch-off hot stamping foils, either printed or scratch-off holographic foil, applied and transferred over the number to form a thin frangible layer where the hologram is destroyed by tampering and where the holographic scratch foil is opaque to read-through by optical techniques relying on the transmission of high intensity light sources. Scratch-off hot stamping foils including holographic scratch off foils are routinely commercially available from a number of sources including Leonhard Kurz GmbH and Optaglio Limited and these devices typically include a holographic layer, a heat activated soft scratch off adhesive designed to be heat applied to the card substrate under typical hot foil stamping parameters. In these scratch-off holographic foils, the adhesive can be high opacity (black) using typically graphite powders or lower opacity (white, using alternative fillers) to reduce smear as the adhesive breaks up on removal as noted and described in WO 02/093474 A1. Often the aluminium reflective layer is used to provide enhanced opacity and is produced by vacuum metallising relatively thick layers of aluminium to increase the light blocking capability. Multi layer scratch off foils (where different adhesive or foil layers have different properties) are well known in the general foil and hot stamping foils field—for example banknote or cards grades of hot stamping contain scratch resistant hard layers, emboss lacquer layers and then optionally hard protective layers underneath the emboss and metallization layers and then often a multi-layer adhesive consisting of a first, or primer, layer and a main heat-activated adhesive layer. A commercially available holographic scratch off foil from Leonhard Kurz comprises a layer structure containing an emboss lacquer, metallisation layer then a layered adhesive consisting of a soft easily removed scratch off layer, followed by a hard durable layer to protect the underlying data from damage followed by a heat activated adhesive layer to bind the structure onto the PIN panel area during transfer.
One known form of attempted number-compromise of scratch cards, such as phone-cards, is to use an intense light source which in some cases can allow the numbers to be read in transmission. This is regarded as a serious form of potential compromise since the secure number potentially can be read and used non-destructively and then the card sold on as genuine.
It is known in the field to produce improved scratch foils with high opacity by using very high opacity metallised layers typically aluminium, sometimes produced by multiple passing of the foil through a metalliser to increase the overall thickness of the aluminium layer to increase the opacity. It is known by this method to produced enhanced opacity scratch foils to defeat light transmission compromise and it is also known to conduct forensic tests that will measure the resistance of the card to light transmission typically conducted on flat cards.
A typical manufacturing technique for a current generation high opacity foil involves embossing a holographic hot stamping foil, aluminising the foil in a vacuum metalliser to give a high opacity aluminium layer and then coating the foil with scratch-off adhesive in a wet coating machine using typical solvent and other coating techniques such as meyer bar, direct gravure and reverse gravure. For more advanced structures the vacuum coating can potentially place an aluminium layer to increase the opacity of the foil, and the wet coating process potentially layering the scratch-off adhesive layers to give various properties on the scratch foil layer. This material would typically be slit and then applied by a process of hot stamping—which releases the hot stamped release layer from the PET carrier to transfers the scratch adhesive layer and the aluminium layer and holographic emboss lacquer and image onto a substrate to form a thin holographic scratch-off coating over a PIN number.
There is an additional light transmission test, or potential attack, that involves the card substrate being additionally bent (distorted). In some cases with scratch foils when the card and scratch foil is bent and curved and stretched (typically the card is curved to stretch and crack the metal reflector of the holographic scratch-off coating) causing a set of micro-cracks form in the aluminium layer can reduce the opacity of the metal layer and thus allow light leakage and visualisation of the data. Thicker aluminium films created by multiple aluminium coatings tend to increase the overall opacity but may still tend to become cracked and crazed when sufficiently stretched by card distortion. This means that single layer aluminium layers can be prone to reduced opacity under bending by micro cracking the aluminium layer to allow a small amount of light transmission.